Phosphoric acid, an important ingredient of chemical fertilizers, is produced from natural deposits of phosphate rock by the so-called wet-process, in which ground phosphate rock is reacted with sulfuric acid to produce phosphoric acid as a solution and gypsum as a precipitate.
The composition of the reactor slurry in phosphoric acid production processes varies somewhat but such slurries typically contain ground rock, about 33% phosphorous pentoxide (equivalent to about 45.55% phosphoric acid), 2 to 5% sulfuric acid, 1 to 3% fluosilicic acid, fluosilicates and small amounts of chlorides and hydrofluoric acid. The operating temperature is typically about 80.degree. C.
Metallic equipment for handling phosphoric acid reactor slurry is subjected to scouring or abrasive action of the suspended solid particles as well as to chemical attack by the acid solution. Pump parts, elbows and other cast shapes are particularly susceptible to damage.
Stainless steels and nickel-chromium corrosion resistant alloys have been used for phosphoric acid reactor equipment. Such alloys have been hardened by cold working, phase transformation of the metallic matrix, precipitation of hard carbides, or precipitation of other hard phases including borides, silicides and sigma phase. Cold working and deformation, however, do not substantially enhance abrasion resistance. Moreover, cold working and deformation are not applicable to cast shapes. Alloys which are hardened with significant amounts of borides, silicides and sigma phase have generally been quite brittle due to the brittle nature of these phases.
Alloys previously formulated for service in abrasive, erosive or corrosive environments include Illium B, Illium P, Lewmet, HC250 and SPA, but these alloys have not provided satisfactory performance in phosphoric reactors and typically only provide a service life of about two to four months. There remains, therefore, a need for an improved alloy to handle both the corrosive and the abrasive actions of phosphoric acid slurries. Since phosphoric acid processes employ large quantities of sulfuric acid, it is desirable for the selected alloy to also be resistive to that acid.